Plate-type heat exchanger

ABSTRACT

A plate-type heat exchanger includes a plurality of heat exchange units stacked on each other. Adjacent heat exchange units are disposed in such a manner that a projection plane of a through hole in one heat exchange unit does not overlap a through hole in another heat exchange unit as seen from a direction of a flow passage for a second fluid. The plurality heat exchange units includes a heat exchange unit having, on the projection plane, a height varying portion varying a height of a flow passage for a first fluid; and a heat exchange unit having, on the projection plane, a planar portion making the height of the flow passage for the first fluid substantially constant.

FIELD OF THE INVENTION

The present invention relates to a plate-type heat exchanger formed bystacking a plurality of heat exchange units, each of which beingconfigured to exchange heat between a first fluid flowing inside theheat exchange unit and a second fluid flowing outside the heat exchangeunit.

DESCRIPTION OF THE RELATED ART

Conventionally, a plate-type heat exchanger having a plurality of heatexchange units in which an upper heat exchange plate and a lower heatexchange plate are joined has been proposed (for example, Patent PriorArt 1: KR 10-1608149 A). Each of the heat exchange units has an internalspace through which a heat medium as a first fluid flows between theupper heat exchange plate and the lower heat exchange plate, and aplurality of through holes penetrating the internal space in anon-communicating state and through which combustion exhaust gas as asecond fluid passes in a vertical direction.

The plate-type heat exchanger includes a plurality of blocks stacked ontop of each other and each of the blocks includes at least one heatexchange unit. Further, adjacent blocks in the vertical directioncommunicate with each other in such a manner that the heat medium flowstherethrough. Further, the adjacent blocks are formed in such a mannerthat a heat medium flow passage in one block is different indirection tothat in another block. According to this configuration, the heat mediumflow passage in the heat exchanger becomes longer as the number ofblocks increases, leading to improvement in heat efficiency.

In the heat exchange unit having the through hole penetrating theinternal space in the non-communicating state as described above, aperipheral portion of the through hole through which the combustionexhaust gas passes is most heated. Therefore, in order to enhancethermal efficiency, a structure of the heat exchange unit in which asmuch heat of the combustion exhaust gas as possible is efficientlytransferred to the heat medium near the peripheral portion of thethrough hole is preferable.

Further, in the plate-type heat exchanger formed by stacking theplurality of heat exchange units, adjacent heat exchange units arepreferably disposed in such a manner that a projection plane of athrough hole in one heat exchange unit does not overlap a through holein another heat exchange unit as seen from a direction of a gas flowpassage of the combustion exhaust gas. The gas flow passage of thecombustion exhaust gas in the heat exchanger thus becomes long, leadingto improvement in heat efficiency.

However, in the heat exchanger having the through hole arrangementstructure described above, a through hole in an upstream-side heatexchange unit faces a projection plane where a through hole in adownstream-side heat exchange unit is not located. Therefore, when thecombustion exhaust gas passes through the through hole in theupstream-side heat exchange unit, then the combustion exhaust gascollides with the projection plane on a surface of the downstream-sideheat exchange unit. Thereafter, the combustion exhaust gas spreads overthe surface of the downstream-side heat exchange unit, and then flows toa downstream side of the gas flow passage of the combustion exhaust gasthrough the through hole in the downstream-side heat exchange unit.Accordingly, in an upstream region of the gas flow passage of thecombustion exhaust gas, high-temperature combustion exhaust gasconcentratedly heats a small portion of the downstream-side heatexchange unit facing the through hole in the upstream-side heat exchangeunit, that is, the projection plane in the downstream-side heat exchangeunit, resulting in local heating. In particular, the high-temperaturecombustion exhaust gas which flows through a through hole in a mostupstream heat exchange unit and does not exchange heat with the heatmedium flowing inside the most upstream heat exchange unit collides witha downstream-side heat exchange unit adjacent to the most upstream heatexchange unit located on a most upstream side of the gas flow passage ofthe combustion exhaust gas. As a result, the local heating is likely tooccur at the downstream-side heat exchange unit adjacent to the mostupstream heat exchange unit.

SUMMARY OF THE INVENTION

The present invention has been made to solve the problem describedabove, and an object of the present invention is to improve heatefficiency and to prevent local heating at a heat exchange unit in anupstream region of a flow passage of a second fluid.

According to the present invention, there is provided a plate-type heatexchanger comprising a plurality of heat exchange units stacked on eachother,

wherein each heat exchange unit is configured to exchange heat between afirst fluid flowing inside the heat exchange unit and a second fluidflowing outside the heat exchange unit,

each heat exchange unit has a plurality of through holes allowing thesecond fluid to flow outside the heat exchange units in a directionintersecting a flow passage plane of the first fluid flowing inside theheat exchange units,

adjacent heat exchange units are disposed in such a manner that aprojection plane of the through hole in one heat exchange unit does notoverlap the through hole in another heat exchange unit as seen from adirection of a flow passage for the second fluid,

the plurality of heat exchange units includes: a height varyingportion-equipped heat exchange unit (P) having, on the projection plane,a height varying portion varying a height of a flow passage for thefirst fluid; and a planar portion-equipped heat exchange unit (Q)having, on the projection plane, a planar portion making the height ofthe flow passage for the first fluid substantially constant, and

at least a second heat exchange unit that is adjacent to and located ona downstream side of a most upstream heat exchange unit located on amost upstream side of the flow passage for the second fluid, includesthe planar portion-equipped heat exchange unit (Q).

Other objects, features and advantages of the present invention willbecome more fully understood from the detailed description givenhereinbelow and the accompanying drawings which are given by way ofillustration only, and thus are not to be considered as limiting thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partial cut-away perspective view showing a heatsource device having a heat exchanger according to an embodiment of thepresent invention;

FIG. 2 is a schematic partial exploded perspective view showing the heatexchanger according to the embodiment of the present invention;

FIG. 3 is a schematic diagram showing flows of a first fluid and asecond fluid in the heat exchanger according to the embodiment of thepresent invention;

FIG. 4 is a schematic partial exploded perspective view showing the heatexchanger according to the embodiment of the present invention;

FIG. 5 is a schematic plan view showing one example of an upper surfaceof one heat exchange plate forming a height varying portion-equippedheat exchange unit (P) in the heat exchanger according to the embodimentof the present invention;

FIG. 6 is a schematic plan view showing one example of an upper surfaceof another heat exchange plate forming the height varyingportion-equipped heat exchange unit (P) in the heat exchanger accordingto the embodiment of the present invention;

FIG. 7 is a schematic exploded perspective view showing a most upstreamheat exchange unit and a second heat exchange unit in the heat exchangeraccording to the embodiment of the present invention;

FIG. 8 is a schematic plan view showing one example of an upper surfaceof one heat exchange plate forming a planar portion-equipped heatexchange unit (Q) in the heat exchanger according to the embodiment ofthe present invention;

FIG. 9 is a schematic plan view showing one example of an upper surfaceof another heat exchange plate forming the planar portion-equipped heatexchange unit (Q) in the heat exchanger according to the embodiment ofthe present invention; and

FIG. 10 is a schematic partial cross-sectional view showing the heatexchanger according to the embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, referring to drawings, a heat exchanger and a heat sourcedevice using thereof according to an embodiment of the present inventionwill be described in detail.

As illustrated in FIG. 1, a heat source device according to the presentembodiment is a water heater that heats water (first fluid) flowing intoa heat exchanger 1 through an inlet pipe 20, with combustion exhaust gas(second fluid) generated in a burner 31, and supplies the heated waterto a hot water supplying terminal (not illustrated) such as a faucet ora shower head through an outlet pipe 21. Although not shown, the waterheater is accommodated in an outer casing. Other heating medium (forexample, an antifreezing fluid) as the first fluid may be used.

The water heater includes a burner body 3 constituting an outer shell ofthe burner 31, a combustion chamber 2, the heat exchanger 1, and a drainreceiver 40 that are disposed in this order from the top. Additionally,a fan case 4 housing a combustion fan for feeding a mixture gas of fuelgas and air into the burner body 3 is disposed on one side (a right sidein FIG. 1) of the burner body 3. Further, an exhaust duct 41communicating with the drain receiver 40 is disposed on another side (aleft side in FIG. 1) of the burner body 3. The combustion exhaust gasflowing out to the drain receiver 40 is discharged to an outside of thewater heater through the exhaust duct 41.

In this specification, when the water heater is viewed in a state wherethe fan case 4 and the exhaust duct 41 are disposed on the sides of theburner body 3, a depth direction corresponds to a front-rear direction,a width direction corresponds to a left-right direction, and a heightdirection corresponds to a vertical direction.

The burner body 3 has a substantially oval shape in a plane view. Theburner body 3 is made of stainless steel-based metal, for example.Although not shown, the burner body 3 opens downward.

An introducing unit communicating with the fan case 4 projects upwardfrom a center of the burner body 3. The burner body 3 includes a flatburner 31 having a downward combustion surface 30. The mixture gas issupplied to the burner body 3 by rotating the combustion fan.

The burner 31 is of all primary air combustion type. The burner 31includes a ceramic combustion plate having many flame ports openingdownwardly (not shown) or a combustion mat made by knitting metal fabricwoven like net. The mixture gas supplied into the burner body 3 isjetted downward from the downward combustion surface 30 by supplypressure of the combustion fan. By igniting the mixture gas, flame isformed on the combustion surface 30 of the burner 31 and the combustionexhaust gas is generated. Therefore, the combustion exhaust gas ejectedfrom the burner 31 is fed to the heat exchanger 1 via the combustionchamber 2. Then, the combustion exhaust gas having passed through theheat exchanger 1 passes through the drain receiver 40 and the exhaustduct 41 and is discharged to the outside of the water heater.

In other words, in the heat exchanger 1, an upper side where the burner31 is provided corresponds to an upstream side of a gas flow passage ofthe combustion exhaust gas, and a lower side opposite to the sideprovided with the burner 31 corresponds to a downstream side of the gasflow passage of the combustion exhaust gas.

The combustion chamber 2 has a substantially oval shape in a plane view.The combustion chamber 2 is made of stainless steel-based metal, forexample. The combustion chamber 2 having an upper opening and a loweropening is formed by bending one single metal plate having asubstantially rectangular shape and joining both ends thereof.

As illustrated in FIG. 2, the heat exchanger 1 has a substantially ovalshape in a plane view. The heat exchanger 1 is of the plate-type heatexchanger formed by stacking a plurality of (in this embodiment,thirteen) thin plate heat exchange units 10. The heat exchanger 1 mayhave a housing surrounding an outer circumference thereof.

As illustrated in FIGS. 2 to 4, the heat exchanger 1 includes aplurality of blocks 5 (in this embodiment, four blocks) stacked on topof each other. Each of the blocks 5 includes one or more heat exchangeunits 10. In the following, the blocks 5 will be simply referred to a“block 5” as a generic term. In addition, in accordance with the gasflow passage of the combustion exhaust gas, an uppermost block 5 will bereferred to as a “most upstream block 5 a”. An upper one of the middleblocks 5 will be referred to as a “first downstream-side block 5 b”, anda lower one of the middle blocks 5 will be referred to as a “seconddownstream-side block 5 c” in order from the upstream side. A lowermostblock 5 will be referred to as a “most downstream block 5 d”. The mostupstream block 5 a and first downstream-side block 5 b are formed of oneheat exchange unit 10, respectively. The second downstream-side block 5c is formed of five heat exchange units 10 stacked on each other, andthe most downstream block 5 d is formed of six heat exchange units 10stacked on each other. The heat exchanger 1 may have three or less orfive or more blocks 5. As described later, in a case where one block 5includes a plurality of heat exchange units 10, water flow passages inthe respective heat exchange units 10 of the block 5 extend in parallelin such a manner that the water flows in the same direction. In eachblock 5, adjacent two of heat exchange units 10 communicate with eachother in such a manner that the water flows upward from below. Further,adjacent two of blocks 5 communicate with each other such that the waterflows upward from below. Further, in the adjacent blocks 5, the waterflow passage in each heat exchange unit 10 of one block 5 is opposite indirection to the water flow passage in each heat exchange unit 10 ofanother block 5. Therefore, the water flow passage in each block 5 isfolded back between the adjacent blocks 5 in such a manner that the heatexchanger 1 includes 4 passages (4-PASS) in accordance with the numberof blocks 5. As a result, the water flow passage in the heat exchanger 1becomes longer, resulting in improvement in heat efficiency.

Next, a configuration of the heat exchanger 1 will be described. Asillustrated in FIGS. 2 to 9, the heat exchange units 10 respectivelyhave a common configuration (e.g., a size, a shape). However, the heatexchange units 10 of the second downstream-side block 5 c and mostdownstream block 5 d are different in configuration (e.g., a shape of athrough hole, presence or absence of a height varying portion) from theheat exchange units 10 of the most upstream block 5 a and firstdownstream-side block 5 b as will be described later. In the following,the heat exchange units 10 will be simply referred to a “heat exchangeunit 10” as a generic term. The heat exchange units 10 which have heightvarying portions to be described later and constitute the seconddownstream-side block 5 c and most downstream block 5 d will be simplyreferred to a “heat exchange unit (P)” as a generic term. The heatexchange units 10 which have planar portions to be described later andconstitute the most upstream block 5 a and first downstream-side block 5b will be simply referred to a “heat exchange unit (Q)” as a genericterm. Therefore, the configuration of the heat exchange unit (P) will bedescribed first, and a description on the heat exchange unit (Q) will bemainly given of a difference from the configuration of the heat exchangeunit (P). For clarity sake, the dimensions of elements which arerepresented in the figures do not correspond to the actual dimensions,and do not limit the embodiment.

In the second downstream-side block 5 c and most downstream block 5 d,each heat exchange unit (P) is formed by superimposing a set of upperand lower heat exchange plates 11, 12 in the vertical direction andjoining predetermined positions to be described later with brazingmaterial or the like. The upper and lower heat exchange plates 11, 12 ofeach heat exchange unit (P) have a common configuration, except for partof configuration such as positions of upper and lower through holes, andpresence or absence of water passage holes at corners. As illustrated inFIGS. 4 to 6, the upper and lower heat exchange plates 11, 12 of theheat exchange unit (P) respectively have a substantially oval shape in aplane view. The upper and lower heat exchange plates 11, 12 are made ofstainless steel-based metal, for example. The upper and lower heatexchange plate 11, 12 respectively have a number of substantiallycircular upper and lower through holes 11 a, 12 a on substantiallyentire surface thereof except for the corners. The upper and lower heatexchange plates 11, 12 also have upper and lower through hole flangeportions 11 c, 12 c at peripheral portions of the upper and lowerthrough holes 11 a, 12 a. The upper and lower through holes 11 a, 12 amay have other shapes such as a substantially ellipse or rectangularshape, respectively.

On peripheral edges of the upper and lower heat exchange plates 11, 12,upper and lower peripheral edge joints W1, W2 projecting upward arerespectively formed. The lower peripheral edge joint W2 of the lowerheat exchange plate 12 is set in such a manner that when the lowerperipheral edge joint W2 and a bottom surface peripheral edge of theupper heat exchange plate 11 are joined together, the upper and lowerheat exchange plates 11, 12 are spaced from each other at a gap with apredetermined height.

Further, the upper peripheral edge joint W1 of the upper heat exchangeplate 11 is set in such a manner that when the upper peripheral edgejoint W1 and a bottom surface peripheral edge of the lower heat exchangeplate 12 of an upward adjacent heat exchange unit (P) are joinedtogether, the upper heat exchange plate 11 of a lower heat exchange unit(P) and the lower heat exchange plate 12 of the upward adjacent upperheat exchange unit (P) are spaced from each other at a gap with apredetermined height.

Therefore, by joining the lower peripheral edge joint W2 of the lowerheat exchange plate 12 and the bottom surface peripheral edge of theupper heat exchange plate 11, an internal space 14 having apredetermined height is formed (see FIG. 3). In addition, by joining aplurality of heat exchange units (P), an exhaust space 15 having apredetermined height is formed between vertically adjacent heat exchangeunits (P) (see FIG. 3).

The upper and lower through holes 11 a, 12 a are bored in a latticepattern at predetermined intervals in the front-rear and left-rightdirections over substantially the entire surfaces of the upper and lowerheat exchange plates 11, 12 of the heat exchange units (P) except forfour corners. Further, the upper and lower through hole flange portions11 c, 12 c respectively extend circumferentially outwardly andsubstantially horizontally from an open edge of the corresponding upperand lower through holes 11 a, 12 a, and have a substantially regularoctagonal contour in a plane view. In the present embodiment, the upperand lower through holes 11 a, 12 a have the same size and shape.However, a pair of upper and lower through holes 11 a, 12 a may bedifferent in size and shape from a different pair of upper and lowerthrough holes 11 a, 12 a as long as one pair of the upper and lowerthrough holes 11 a, 12 a opposed to each other in the vertical directionhas the same size and shape.

The upper and lower through holes 11 a, 12 a and the upper and lowerthrough hole flange portions 11 c, 12 c are formed at positionscorresponding to each other in the vertical direction when the upper andlower heat exchange plates 11, 12 are superimposed with each other.Further, the upper and lower through holes 11 a, 12 a and the upper andlower through hole flange portions 11 c, 12 c are respectively formed,by drawing, on a bottom of a step portion projecting inward in such amanner that the facing upper and lower through hole flange portions 11c, 12 c come into surface contact with each other when the upper andlower heat exchange plates 11, 12 are superimposed with each other.

Therefore, when the upper and lower heat exchange plates 11, 12 aresuperimposed with each other and the upper and lower through hole flangeportions 11 c, 12 c are joined by brazing material or the like, flangeportions 16 closing the internal space 14 are formed by the upper andlower through hole flange portions 11 c, 12 c (see FIG. 10). Further,through holes 13 penetrating the internal space 14 in anon-communicating state are formed by the upper and lower through holes11 a, 12 a.

Substantially circular upper and lower recesses 11 b, 12 b arerespectively formed between four upper and lower through holes 11 a, 12a adjacent to each other in the front-rear and left-right directions.Further, in peripheral regions of the upper and lower heat exchangeplates 11, 12, substantially ellipse upper and lower recesses 11 b, 12 bare respectively formed between two upper and lower through holes 11 a,12 a adjacent to each other in the front-rear or left-right direction.Further, upper and lower protrusions 11 d, 12 d are formed atsubstantially centers of the upper and lower recesses 11 b, 12 b,respectively. A diameter of each of the upper and lower protrusions 11d, 12 d is smaller than that of the corresponding upper and lowerrecesses 11 b, 12 b. The upper and lower recesses 11 b, 12 b and theupper and lower protrusions 11 d, 12 d are respectively formed atpositions corresponding to each other in the vertical direction when theupper and lower heat exchange plates 11, 12 are superimposed with eachother. Therefore, the upper and lower recesses 11 b, 12 b and the upperand lower protrusions 11 d, 12 d are respectively formed in a latticepattern at predetermined intervals in the front-rear and left-rightdirections over substantially the entire surfaces of the upper and lowerheat exchange plates 11, 12 except for the four corners. Further, theintervals in the front-rear and left-right directions between adjacentupper and lower recesses 11 b, 12 b are respectively set to besubstantially the same as those between adjacent upper and lower throughholes 11 a, 12 a. Thus, the upper and lower through holes 11 a, 12 a andthe upper and lower recesses 11 b, 12 b are alternately formed atsubstantially equal intervals in the front-rear and left-rightdirections. Further, the upper and lower recesses 11 b, 12 b are formedin such a manner that the upper and lower recesses 11 b, 12 b arelocated at substantially centers of regions surrounded by the fouradjacent upper and lower through holes 11 a, 12 a in the front-rear andleft-right directions except for the peripheral regions of the upper andlower heat exchange plates 11, 12. Further, each of the upper and lowerrecesses 11 b, 12 b has a diameter smaller than a minimum distancebetween the two adjacent upper and lower through hole flange portions 11c, 12 c in the front-rear and left-right directions.

Each of the upper and lower recesses 11 b, 12 b is formed by drawing soas to project by a predetermined height inwardly of the internal space14 when the upper and lower heat exchange plates 11, 12 are superimposedwith each other. Each of the upper and lower recesses 11 b, 12 b is setto be a lower inwardly projecting height than each of the upper andlower through hole flange portions 11 c, 12 c. Each of the upper andlower protrusions 11 d, 12 d is formed by drawing so as to project by apredetermined height outwardly of the internal space 14 when the upperand lower heat exchange plates 11, 12 are superimposed with each other.Each of the upper and lower protrusions 11 d, 12 d is set to be a loweroutwardly projecting height than the upper and lower peripheral edgejoints W1, W2. Therefore, when the upper and lower heat exchange plates11, 12 are superimposed with each other, a height varying portion 17decreasing the height of the internal space 14 is formed by thecorresponding upper and lower recesses 11 b, 12 b and an narrow internalspace 14 having a predetermined height is defined between the upper andlower recesses 11 b, 12 b (see FIG. 10). Further, a height varyingportion 18 increasing the height of the internal space 14 is formed bythe corresponding upper and lower protrusions 11 d, 12 d formed at thesubstantially centers of the upper and lower recesses 11 b, 12 b and awide internal space 14 having a predetermined height is defined betweenthe upper and lower protrusions 11 d, 12 d (see FIG. 10). Although notillustrated, the water flow passage is formed between the height varyingportion 17 and an adjacent flange portion 16. The upper and lowerrecesses 11 b, 12 b and the upper and lower protrusions 11 d, 12 d mayhave other shapes such as a substantially ellipse or rectangular shape,respectively. Further, only one of the height varying portions 17, 18may be formed.

In the heat exchange unit (P), each of the upper and lower heat exchangeplates 11, 12 has a water passage hole 63 in at least one corner. At aperipheral portion of the water passage hole 63, a water passage holeflange portion extends circumferentially outwardly and substantiallyhorizontally from an open end of the water passage hole 63. The waterpassage hole 63 provided at at least one corner of the upper and lowerheat exchange plates 11, 12 forming one heat exchange unit (P) is openedso as to communicate with the internal space 14 between the upper andlower heat exchange plates 11, 12 when the upper and lower heat exchangeplates 11, 12 are superimposed with each other.

As illustrated in FIGS. 2, 3, 7, 8, and 9, the heat exchange unit (Q)has the same configuration as the heat exchange unit (P), except thatupper and lower through holes 11 a, 12 a, upper and lower through holeflange portions 11 c, 12 c in and on the heat exchange unit (Q) aredifferent in shape from those in and on the heat exchange unit (P), thatneither a recess nor a protrusion is formed in regions surrounded withfour upper and lower through holes 11 a, 12 a adjacent to each other inthe front-rear and left-right directions, that no recess is formedbetween two upper and lower through holes 11 a, 12 a adjacent to eachother in the front-rear or left-right direction in peripheral regions ofthe upper and lower heat exchange plates 11, 12, and that the upper heatexchange plate 11 of a most upstream heat exchange unit (Q) has no waterpassage hole. In the most upstream block 5 a and first downstream-sideblock 5 b, each heat exchange unit (Q) is formed by superimposing a setof upper and lower heat exchange plates 11, 12 in the vertical directionand joining predetermined positions to be described later with brazingmaterial or the like. The upper and lower heat exchange plates 11, 12 ofeach heat exchange unit (Q) in the most upstream block 5 a and firstdownstream-side block 5 b have a common configuration, except for partof configuration such as positions of upper and lower through holes 11a, 12 a, and presence or absence of water passage holes 63 at thecorners. Therefore, by joining the upper and lower heat exchange plates11, 12 of each heat exchange unit (Q), an internal space 14 having apredetermined height is formed (see FIG. 3). In addition, by joining aplurality of heat exchange units (Q), an exhaust space 15 having apredetermined height is formed between vertically adjacent heat exchangeunits (Q) (see FIG. 3). Further, by joining the heat exchange unit (P)and the heat exchange unit (Q), an exhaust space 15 having apredetermined height is formed between the heat exchange unit (P) andthe heat exchange unit (Q) adjacent to each other in the verticaldirection (see FIG. 3).

In the heat exchange unit (Q), the upper and lower heat exchange plates11, 12 respectively have a number of substantially square upper andlower through holes 11 a, 12 a formed on substantially the entiresurface thereof except for the corners and peripheral regions. The upperand lower heat exchange plates 11, 12 also have substantially squareupper and lower through hole flange portions 11 c, 12 c formed atperipheral portions of the substantially square upper and lower throughholes 11 a, 12 a. The upper and lower heat exchange plates 11, 12respectively have a plurality of substantially pentagonal upper andlower through holes 11 a, 12 a in the peripheral regions thereof. Theupper and lower heat exchange plates 11, 12 also have substantiallypentagonal upper and lower through hole flange portions 11 c, 12 cformed on peripheral portions of the substantially pentagonal upper andlower through holes 11 a, 12 a. The upper and lower through holes 11 a,12 a may have other shapes such as a substantially circular or ellipseshape, respectively. The upper and lower through holes 11 a, 12 a andthe upper and lower through hole flange portions 11 c, 12 c are formedat substantially the same pitches as those of the heat exchange unit(P). Therefore, when the upper and lower heat exchange plates 11, 12 arejoined together, substantially square through holes 13 and substantiallysquare flange portions 16 closing the internal space 14 are formed at aregion except for the peripheral region of the heat exchange unit (Q).Further, substantially pentagonal through holes 13 and substantiallypentagonal flange portions 16 closing the internal space 14 are formedin the peripheral region of the heat exchange unit (Q). Unlike the heatexchange unit (P), the heat exchange unit (Q) has neither a recess nor aprotrusion in a region surrounded with four through holes 13. Therefore,the heat exchange unit (Q) has a planar portion 19 making the height ofthe internal space 14 substantially constant and being located betweenthe four through holes 13 adjacent to each other in the front-rear andleft-right directions.

Each of the through holes 13 in the region except for the peripheralregion of the heat exchange unit (Q) is disposed in such a manner thatfour vertexes are respectively directed to front, rear, left, and rightsides of the peripheral edge of the heat exchange unit (Q), and one sideof each through hole 13 extends in substantially parallel to one side ofthe through hole 13 diagonally adjacent thereto. Therefore, each throughhole 13 is arranged in such a manner that the vertexes thereof protrudeto the region surrounded with the four through holes 13 adjacentthereto, as seen from the direction of the gas flow passage of thecombustion exhaust gas. Further, each flange portion 16 is formed insuch a manner that each of the four vertexes (e.g., the right vertex) ofthe flange portion 16 is located closer to a center of the through hole13 diagonally adjacent thereto than an opposite vertex (e.g., the leftvertex) of the flange portion 16 diagonally adjacent thereto. In otherword, each flange portion 16 is arranged so as to partially overlap thediagonally adjacent flange portions 16 as seen from the front-rear andleft-right directions of the heat exchange unit (Q).

The water passage hole 63 provided at at least one corner of the upperand lower heat exchange plates 11, 12 forming one heat exchange unit (Q)is opened so as to communicate with the internal space 14 between theupper and lower heat exchange plates 11, 12 when the upper and lowerheat exchange plates 11, 12 are superimposed with each other.

As illustrated in FIG. 3, the heat exchange units (P), (Q) are arrangedin such a manner that, as to adjacent two of the heat exchange units 10,the through hole 13 in one heat exchange unit 10 is shifted from thethrough hole 13 in another heat exchange unit 10 in a lateral directionperpendicularly intersecting the direction of the gas flow passage ofthe combustion exhaust gas. In other word, the vertically adjacent heatexchange units 10 are disposed in such a manner that a projection planeof the through hole 13 in the one heat exchange unit 10 does not overlap the through hole 13 in the other heat exchange unit 10. Therefore,the combustion exhaust gas flowing from the upstream side passes throughthe through hole 13 in the one heat exchange unit 10, and then flows outto the exhaust space 15 between the one heat exchange unit 10 and thedownstream adjacent heat exchange unit 10. Then, the combustion exhaustgas flowing out to the exhaust space 15 collides with the upper heatexchange plate 11 of the downstream adjacent heat exchange unit 10 andfurther flows from the through hole 13 in the downward adjacent heatexchange unit 10 toward the downstream side. Namely, when the combustionexhaust gas flows from the upstream side toward the downstream side inthe heat exchanger 1, a zigzag-shaped gas flow passage is formed in theheat exchanger 1. As a result, in the heat exchanger 1, a contact timebetween the combustion exhaust gas and the upper and lower heat exchangeplates 11, 12 increases. Further, each of the height varying portion 17of the heat exchange unit (P), the height varying portion 18 of the heatexchange unit (P), and the planar portion 19 of the heat exchange unit(Q) is located on a projection plane 55 of the corresponding throughhole 13 in the adjacent heat exchange unit 10 (see FIG. 10). Therelationship between the through holes 13 in the heat exchange unit (P)and the through holes 13 in the heat exchange unit (Q) is similar tothat described above (see FIGS. 3 and 10).

With reference to FIG. 3, next, a description will be given of the flowsof combustion exhaust gas and water in the heat exchanger 1. Each block5 has an introduction port 71 for introducing water into the block 5,and a lead-out port 72 for leading the water out of the block 5. In eachblock 5, predetermined at least one of the water passage holes 63 in theheat exchange unit 10 located on a most downstream side of the gas flowpassage of the combustion exhaust gas forms the introduction port 71,and predetermined at least one of the water passage holes 63 in the heatexchange unit 10 located on a most upstream side of the gas flow passageof the combustion exhaust gas forms the lead-out port 72. Note that inFIG. 3, the flange portions 16 around the through holes 13 and therecesses and the protrusions are omitted for simplicity of illustration.

In the heat exchange unit (P) located on the most downstream side of thegas flow passage of the combustion exhaust gas (hereinafter, referred toas a “most downstream heat exchange unit 10 s”), the water passage hole63 in a front right corner of the lower heat exchange plate 12 isconnected to the inlet pipe 20. Also, a lead-out pipe 23 is insertedinto the water passage hole 63 in a rear right corner of the lower heatexchange plate 12 in the most downstream heat exchange unit 10 s. Thelead-out pipe 23 extends upward from the most downstream heat exchangeunit 10 s to the heat exchange unit (Q) located on the most upstreamside of the gas flow passage of the combustion exhaust gas (hereinafter,referred to as a “most upstream heat exchange unit 10 a”). An upper endof the lead-out pipe 23 is connected to the water passage hole 63 in arear right corner of the lower heat exchange plate 12 of the mostupstream heat exchange unit 10 a. An outer peripheral surface of thelead-out pipe 23 and an inner periphery of the water passage hole 63 inthe rear right corner of the lower heat exchange plate 12 of the mostdownstream heat exchange unit 10 s are joined together by a brazingmaterial or the like. An upper end opening of the lead-out pipe 23communicates with the internal space 14 in the most upstream heatexchange unit 10 a. Further, when the lead-out pipe 23 is inserted fromthe most downstream heat exchange unit 10 s to the most upstream heatexchange unit 10 a, the lead-out pipe 23 passes through, in anon-communicating state, all the internal spaces 14 in the heat exchangeunits 10 except for the internal space 14 of the most upstream heatexchange unit 10 a. Further, the lead-out pipe 23 passes through, in anon-communicating state, all the exhaust spaces 15 between the adjacentheat exchange units 10.

Accordingly, when the water flows into the internal space 14 in eachheat exchange unit (P) of the most downstream block 5 d through thewater passage hole 63 in the front right corner, then the waterlaterally flows through the internal space 14 in one direction (fromright to left in FIG. 3). When the water flows into the internal space14 of each heat exchange unit (P) of the second downstream-side block 5c through each of the water passage holes 63 in front and rear leftcorners, then the water laterally flows through the internal space 14 inone direction (from left to right in FIG. 3). The water flow passage inthe internal space 14 in each heat exchange unit (P) of the seconddownstream-side block 5 c is opposite in direction to that of the mostdownstream block 5 d. Further, when the water flows into the internalspace 14 in the heat exchange unit (Q) (hereinafter, referred to as a“second heat exchange unit 10 b”) of the first downstream-side block 5 bthrough the water passage hole 63 in a front right corner, then thewater laterally flows through the internal space 14 in one direction(from right to left in FIG. 3). The water flow passage in the internalspace 14 in the heat exchange unit 10 b is opposite in direction to thatof the second downstream-side block 5 c. Further, when the water flowsinto the internal space 14 in the most upstream heat exchange unit 10 athrough each of the water passage holes 63 in front and rear leftcorners, then the water laterally flows through the internal space 14 inone direction (from left to right in FIG. 3). The water flow passage inthe internal space 14 in the heat exchange unit 10 a is opposite indirection to that of the heat exchange unit 10 b. After the water flowsthrough the internal space 14 in the most upstream heat exchange unit 10a, the water flows into the lead-out pipe 23 connected to the waterpassage hole 63 in the rear right corner of the most upstream heatexchange unit 10 a. When the water flows into the lead-out pipe 23, thenthe water flows downward through the lead-out pipe 23, and flows out ofthe heat exchanger 1 through the outlet pipe 21 connected to the mostdownstream heat exchange unit 10 s. As described above, the mostupstream heat exchange unit 10 a and second heat exchange unit 10 b inan upstream region of the gas flow passage of the combustion exhaust gasare connected in series in such a manner that the whole of water, whichhas flowed into the second heat exchange unit 10 b, flows into the mostupstream heat exchange unit 10 a. In addition, the heat exchange units(P) of the most downstream block 5 d are connected in parallel in such amanner that multiple flow passages are formed in parallel. Aconfiguration of the second downstream-side block 5 c is similar to thatof the most downstream block 5 d.

With reference to FIG. 10, next, a description will be given of the flowof combustion exhaust gas in the upstream region of the gas flow passageof the combustion exhaust gas and the flow of water in the internalspace 14 in each of the heat exchange units (P), (Q). Note that FIG. 10is a partial cross-sectional view of the heat exchange units (P), (Q)taken along an inclined direction at a certain degree with respect tothe front-rear and left-right directions so as to make the differencebetween the heat exchange units (P), (Q) clear.

The water flows between laterally distant water passage holes 63 in eachof the heat exchange units (P), (Q). The heat exchange unit (P) has theheight varying portions 17, 18 where the height of the water flowpassage in the region surrounded with the through holes 13 increases anddecreases. Therefore, when the water flowing from the upstream side ofthe internal space 14 passes the height varying portions 17, 18, theheight varying portions 17, 18 increase flow resistance of the water toreduce a flow rate of the water. In addition, when the water passes theheight varying portions 17, 18, the height varying portions 17, 18 causeturbulence of the water to narrow a temperature distribution of thewater. Further, since the height varying portions 17, 18 increase asurface area of the heat exchange unit (P), the heat exchange unit (P)has a larger heat receiving area. According to this configuration, heatreceived from the combustion exhaust gas can be efficiently transferredto the water at the downstream side of the gas flow passage of thecombustion exhaust gas. Furthermore, since the heat exchange units (P)excellent in heat transferability are stacked on the downstream side ofthe heat exchange unit (Q), the upstream-side heat exchange unit (Q)absorbs sensible heat of the high-temperature combustion exhaust gas,and the downstream-side heat exchange unit (P) absorbs latent heat ofthe combustion exhaust gas. This configuration can thus improve heatefficiency.

On the other hand, in the present embodiment, the combustion exhaust gasflows vertically through the through holes 13 in each heat exchange unit10. In each heat exchange unit 10, therefore, the through holes 13 arebored to allow the combustion exhaust gas to flow outside the heatexchange unit 10 in a direction intersecting substantiallyperpendicularly to a flow passage plane of the water flowing inside theheat exchange unit 10. In addition, the through holes 13 are arranged inthe front-rear and left-right directions at substantially regularspacings over substantially the entire surface of each heat exchangeunit 10. Therefore, the combustion exhaust gas flowing from the upstreamside of the gas flow passage of the combustion exhaust gas collides withthe entire surface of the most upstream heat exchange unit 10 a exceptfor the through holes 13 to heat the most upstream heat exchange unit 10a. In other words, a portion of the most upstream heat exchange unit 10a except for the through holes 13 serves as a heat receiving plane. Asdescribed above, the adjacent heat exchange units 10 are disposed insuch a manner that the projection plane of the through hole 13 in theone heat exchange unit 10 does not over lap the through hole 13 in theother heat exchange unit 10. Therefore, after the combustion exhaust gasflows through each through hole 13 in the most upstream heat exchangeunit 10 a, the combustion exhaust gas concentratedly collides with thecorresponding projection plane 55 having a small area on the second heatexchange unit 10 b. The combustion exhaust gas, which collides with thesecond heat exchange unit 10 b, includes the high-temperature combustionexhaust gas which is not in contact with the most upstream heat exchangeunit 10 a (i.e., the combustion exhaust gas which does not exchange heatwith the water flowing through the internal space 14 in the mostupstream heat exchange unit 10 a). For this reason, if the heatexchanger 1 only consists of the heat exchange unit (P) having theheight varying portions 17, 18, local overheating is likely to occur atthe second heat exchange unit 10 b that is adjacent to and is located onthe downstream side of the most upstream heat exchange unit 10 a.

However, according to the present embodiment, the heat exchange unit (Q)having the planar portion 19 making the height of the water flow passagesubstantially constant is used as the second heat exchange unit 10 b.The planar portion 19 is disposed on the region surrounded with the fourthrough holes 13, that is, the projection plane 55 of the correspondingthrough hole 13 in the most upstream heat exchange unit 10 a (see FIGS.7 to 10). Therefore, the flow passage resistance of the water passingthe projection plane 55 in the heat exchange unit (Q) is smaller thanthat of the water passing the projection plane 55 in the heat exchangeunit (P), leading to increase in the flow rate of the water passing theprojection plane 55 in the heat exchange unit (Q). In addition, sincethe planar portion 19 has no irregularities, the combustion exhaust gascolliding with the planar portion 19 uniformly spreads in alldirections. This results in relief from heat concentration on theprojection plane 55 where the combustion exhaust gas concentratedlycollides. Thus, this configuration can prevent the local overheating atthe second heat exchange unit 10 b.

Further, since the highest-temperature combustion exhaust gas collideswith the most upstream heat exchange unit 10 a, the peripheral portionof each through hole 13 through which the combustion exhaust gas flowsis most heated. Therefore, when the flow rate of the water is low, localoverheating may occur at the most upstream heat exchange unit 10 a.However, according to the present embodiment, the heat exchange unit (Q)is used as the most upstream heat exchange unit 10 a. Therefore, theplanar portion 19 is disposed on the region surrounded with the fourthrough holes 13 in the most upstream heat exchange unit 10 a, that is,the projection plane 55 of the corresponding through hole 13 in thesecond heat exchange unit 10 b. Thus, this configuration can prevent thelocal overheating at the most upstream heat exchange unit 10 a.

Further, according to the present embodiment, the most upstream heatexchange unit 10 a and second heat exchange unit 10 b in the upstreamregion of the gas flow passage of the combustion exhaust gas areconnected in series in such a manner that the water flows through thesecond heat exchange unit 10 b and the most upstream heat exchange unit10 a in this order. With this configuration, the whole of water, whichhas flowed through the second heat exchange unit 10 b, flows into themost upstream heat exchange unit 10 a. This configuration therefore canprevent the local overheating at the second heat exchange unit 10 b andmost upstream heat exchange unit 10 a even when the amount of water tobe supplied to the heat exchanger 1 is small.

Further, according to the present embodiment, the most upstream heatexchange unit 10 a and second heat exchange unit 10 b respectively havethe substantially square through holes 13. Further, the respectivevertexes of each substantially square through hole 13 protrude to theregion surrounded with the substantially square through holes 13adjacent thereto, that is, the projection plane 55 as seen from thedirection of the gas flow passage of the combustion exhaust gas. Thisconfiguration can accelerate the velocity of the water passing theregion surrounded with the substantially square through holes 13. Thus,this configuration can further prevent the local overheating.

Further, according to the present embodiment, the heat exchange unit(Q), that is, each of the most upstream heat exchange unit 10 a andsecond heat exchange unit 10 b has a protrusion protruding outwardbetween adjacent two of the through holes 13 in the peripheral region ofthe heat exchange unit (Q). The projection planes 55 are also formed inthe peripheral region; however, no through hole 13 is bored in aperipheral edge side of each projection plane 55. On the other hand,each of the projection planes 55 except for the projection planes 55 inthe peripheral region of the heat exchange unit (Q) is surrounded withthe four through holes 13. Therefore, the amount of heat received fromcombustion exhaust gas on each projection plane 55 surrounded with thefour through holes 13 is larger than that on each projection plane 55located on the peripheral region, so that local overheating is likely tooccur. Therefore, by disposing the planar portions 19 at least on theprojection planes 55 except for the projection planes 55 located on theperipheral region of the heat exchange unit (Q), the local overheatingcan be effectively prevented. Further, when the irregularities aredisposed between adjacent two of the through holes 13 in the peripheralregion of the heat exchange unit (Q), the heat of the combustion exhaustgas can be efficiently transferred to the water.

According to the present invention, not only heat efficiency can beimproved but local heating at the heat exchange unit in the upstreamregion of the gas flow passage of the combustion exhaust gas can beprevented. Accordingly, the plate-type heat exchanger excellent in heatefficiency and durability can be provided.

OTHER EMBODIMENTS

(1) In the above embodiment, the downstream-side heat exchange unitsdownstream of the second heat exchange unit only consist of the heatexchange unit (P) having the height varying portion. However, if localoverheating occurs at one or more downstream-side heat exchange unitsdownstream of the second heat exchange unit, the heat exchange unit (Q)may be used as part of the downstream-side heat exchange units insteadof the heat exchange unit (P). Further, the heat exchange unit (P) maybe used as the most upstream heat exchange unit instead of the heatexchange unit (Q).

(2) In the above embodiment, the burner having the downward combustionsurface is disposed above the heat exchanger. However, a burner havingan upward combustion surface may be disposed below the heat exchanger.

(3) In the above embodiment, the plurality of heat exchange units isstacked in the vertical direction. However, a plurality of heat exchangeunits may be stacked in the left-right direction.

(4) In the above embodiment, the water heater is used. However, a heatsource device such as a boiler may be used.

As described in detail, the present invention is summarized as follows.

According to the present invention, there is provided a plate-type heatexchanger comprising a plurality of heat exchange units stacked on eachother,

wherein each heat exchange unit is configured to exchange heat between afirst fluid flowing inside the heat exchange unit and a second fluidflowing outside the heat exchange unit,

each heat exchange unit has a plurality of through holes allowing thesecond fluid to flow outside the heat exchange units in a directionintersecting a flow passage plane of the first fluid flowing inside theheat exchange units,

adjacent heat exchange units are disposed in such a manner that aprojection plane of the through hole in one heat exchange unit does notoverlap the through hole in another heat exchange unit as seen from adirection of a flow passage for the second fluid,

the plurality of heat exchange units includes: a height varyingportion-equipped heat exchange unit (P) having, on the projection plane,a height varying portion varying a height of a flow passage for thefirst fluid; and a planar portion-equipped heat exchange unit (Q)having, on the projection plane, a planar portion making the height ofthe flow passage for the first fluid substantially constant, and

at least a second heat exchange unit that is adjacent to and located ona downstream side of a most upstream heat exchange unit located on amost upstream side of the flow passage for the second fluid, includesthe planar portion-equipped heat exchange unit (Q).

According to the heat exchanger described above, the heat exchange unit(P) has the height varying portion where the height of the flow passagefor the first fluid varies, and the height varying portion is located onthe projection plane of a corresponding through hole in an adjacent heatexchange unit. Therefore, flow resistance of the first fluid increasesat the height varying portion, which results in reducing a flow rate ofthe first fluid passing the projection plane. Further, since turbulenceof the first fluid occurs when the first fluid passes the projectionplane, a temperature distribution of the first fluid narrows. Further,since the height varying portion increases a surface area of the heatexchange unit (P), the heat exchange unit (P) has a larger heatreceiving area. According to this configuration, heat received from thesecond fluid can be efficiently transferred to the first fluid.

On the other hand, in an upstream region of the second fluid, ahigh-temperature second fluid flowing through the through hole in themost upstream heat exchange unit concentratedly collides with acorresponding projection plane having a small area on the second heatexchange unit. As described above, when the projection plane has theheight varying portion, the flow rate of the first fluid passing theprojection plane is likely to decrease. For this reason, if the heatexchanger only consists of the heat exchange unit (P) having the heightvarying portion, local overheating is likely to occur at the projectionplane in the second heat exchange unit which corresponds to the throughhole in the most upstream heat exchange unit. However, according to theheat exchanger described above, the second heat exchange unit includesthe heat exchange unit (Q) having the planar portion making the heightof the water flow passage substantially constant, and the planar portionis disposed on the projection plane of the corresponding through hole inthe adjacent heat exchange unit. Therefore, the flow passage resistanceof the first fluid passing the projection plane in the heat exchangeunit (Q) is smaller than that of the first fluid passing the projectionplane in the heat exchange unit (P), leading to increase in the flowrate of the first fluid passing the projection plane in the heatexchange unit (Q). In addition, since the planar portion has noirregularities, the second fluid colliding with the planar portionuniformly spreads in all directions. This results in relief from heatconcentration on the projection plane of the second heat exchange unit.

Preferably, in the heat exchanger described above,

the most upstream heat exchange unit includes the planarportion-equipped heat exchange unit (Q).

According to the heat exchanger described above, local overheating atthe most upstream heat exchange unit where the high-temperature secondfluid collides can be prevented.

Preferably, in the heat exchanger described above,

the planar portion is formed on the projection plane in a region exceptfor a peripheral region of the planar portion-equipped heat exchangeunit (Q).

As to adjacent two of heat exchange units, when the projection plane ofthe corresponding through hole in one heat exchange unit is disposed inthe peripheral region of another heat exchange unit, no through hole isprovided in a peripheral edge side of the projection plane. On the otherhand, the projection plane in the region except for the peripheralregion of the heat exchange unit is surrounded with the four throughholes. Therefore, an amount of heat received from the second fluid onthe projection plane surrounded with the four through holes is largerthan that on the projection plane located on the peripheral region. As aresult, in the upstream region of the second fluid, local overheating islikely to occur at the projection plane in the region except for theperipheral region. Therefore, by disposing the planar portion at leaston the projection plane in the region except for the peripheral regionof the heat exchange unit (Q), the local overheating can be effectivelyprevented.

Preferably, in the heat exchanger described above,

the second heat exchange unit and the most upstream heat exchange unitare connected in series in such a manner that the first fluid passesinside the second heat exchange unit and the most upstream heat exchangeunit in this order.

According to the heat exchanger described above, the whole of water,which has flowed through the second heat exchange unit, flows into themost upstream heat exchange unit. This configuration therefore canprevent the local overheating at the second heat exchange unit and themost upstream heat exchange unit even when the amount of water to besupplied to the heat exchanger is small.

Preferably, in the heat exchanger described above,

the through hole of the planar portion-equipped heat exchange unit (Q)has a substantially rectangular shape, and

the through hole having the substantially rectangular shape is arrangedin such a manner that at least one vertex protrudes to the projectionplane.

According to the heat exchanger described above, the velocity of thefirst fluid passing the projection plane in the heat exchange unit (Q)can be accelerated. Thus, this configuration can further prevent thelocal overheating.

The present application claims a priority based on a Japanese PatentApplication No. 2019-188709 filed on Oct. 15, 2019, the content of whichis hereby incorporated by reference in its entirely.

Although the present invention has been described in detail, theforegoing descriptions are merely exemplary at all aspects, and do notlimit the present invention thereto. It should be understood that anenormous number of unillustrated modifications may be assumed withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A plate-type heat exchanger comprising aplurality of heat exchange units stacked on each other, wherein eachheat exchange unit is configured to exchange heat between a first fluidflowing inside the heat exchange unit and a second fluid flowing outsidethe heat exchange unit, each heat exchange unit has a plurality ofthrough holes allowing the second fluid to flow outside the heatexchange units in a direction intersecting a flow passage plane of thefirst fluid flowing inside the heat exchange units, adjacent heatexchange units are disposed in such a manner that a projection plane ofthe through hole in one heat exchange unit does not overlap the throughhole in another heat exchange unit as seen from a direction of a flowpassage for the second fluid, the plurality of heat exchange unitsincludes: a height varying portion-equipped heat exchange unit (P)having, on the projection plane, a height varying portion varying aheight of a flow passage for the first fluid; and a planarportion-equipped heat exchange unit (Q) having, on the projection plane,a planar portion making the height of the flow passage for the firstfluid substantially constant, and at least a second heat exchange unitthat is adjacent to and located on a downstream side of a most upstreamheat exchange unit located on a most upstream side of the flow passagefor the second fluid, includes the planar portion-equipped heat exchangeunit (Q).
 2. The plate-type heat exchanger according to claim 1, whereinthe most upstream heat exchange unit includes the planarportion-equipped heat exchange unit (Q).
 3. The plate-type heatexchanger according to claim 1, wherein the planar portion is formed onthe projection plane in a region except for a peripheral region of theplanar portion-equipped heat exchange unit (Q).
 4. The plate-type heatexchanger according to claim 1, wherein the second heat exchange unitand the most upstream heat exchange unit are connected in series in sucha manner that the first fluid passes inside the second heat exchangeunit and the most upstream heat exchange unit in this order.
 5. Theplate-type heat exchanger according to claim 1, wherein the through holeof the planar portion-equipped heat exchange unit (Q) has asubstantially rectangular shape, and the through hole having thesubstantially rectangular shape is arranged in such a manner that atleast one vertex protrudes to the projection plane.